Holmgang doesn't last as long as Hallowed, and Holmgang doesn't make you actually any easier to heal. Examples:
Warrior facepulls 30 enemies. He uses Holmgang on one and spams Overpower, but the healer just can't heal through that much output. Warrior never drops below 1hp for the duration, but as soon as it wear off, Warrior eats dirt.
Paladin facepulls 30 enemies. Right before he dies, he pops Hallowed Ground. The healer can heal the paladin to full without worry of him dying, and when it drops the paladin can then hit cooldowns to extend that full HP pool.
Warrior facepulls 30 enemies. He uses Holmgang on one and spams Overpower, but the healer just can't heal through that much output. Warrior never drops below 1hp for the duration, but as soon as it wear off, Warrior eats dirt.
Paladin facepulls 30 enemies. Right before he dies, he pops Hallowed Ground. The healer can heal the paladin to full without worry of him dying, and when it drops the paladin can then hit cooldowns to extend that full HP pool.